This invention relates to a rack and pinion steering gear assembly of the well known kind in which a rack bar is longitudinally displaceable within a tubular housing in response to rotation of a pinion and one or each end of the rack bar has a linkage, usually a tie rod, extending therefrom and from which linkage or linkages a steering output is derived. With a gear of this kind it is conventional to provide a flexible enclosure, usually in the form of a bellows or boot, which extends from the tie rod to the housing to provide a sealed cover therebetween which alleviates the ingress of foreign matter to the interior of the housing (and to a universal joint which is usually provided between the tie rod and the rack bar) and flexes to accommodate longitudinal displacement of the rack bar. During longitudinal displacement of the rack bar the volume within the flexible enclosure changes and accordingly it is necessary to allow the enclosure to breathe and to expel air therefrom, or admit air thereto, as appropriate. Generally the enclosure for a rack and pinion steering gear is in the form of a plastics or rubber moulded concertina-type bellows which is arranged to breather to atmosphere or, in the case of a gear with a tie rod at each end of the rack bar and two bellows (one for each tie rod), from one bellows to the other. The latter arrangement is usually preferred since it permits a fully sealed assembly. It it conventional for the rack bar to be supported for its displacement by one or more bushes mounted within the housing and through which bushes it is longitudinally slidable. With wholly manual rack and pinion gears there is usually little problem in providing communication through the housing between bellows at each end of the rack bar by having slots or conduits through the bush by way of which the bellows can breathe from one to the other, for example as disclosed in G.B. Pat. No. 1,242,813. However, this technique of providing communication between two bellows directly through the housing is not always convenient or possible in the case of power assisted rack and pinion steering gears where it is usual that a hydraulically sealed piston and cylinder unit is formed between the rack bar and its housing (to provide the power assistance) and this unit generally renders impractical the aforementioned communication. Accordingly, where two bellows are provided with an end take-off power assisted rack and pinion gear it is conventional either, for each bellows to breather to atmosphere, or for the bellows to breather from one to the other by way of a pipe or conduit which communicates between the two bellows externally of the housing. U.S. Pat. No. 3,927,576 discloses a bellows which breathes to atmosphere by way of an air filter where the bellows is of a non-standard design or structure to accommodate the filter. G.B. Pat. No. 1,404,986 discloses the manner in which two bellows can breathe from one to the other by way of an external pipe where the bellows are also of non-standard form being specially moulded to permit attachment of the external pipe thereto. G.B. Pat. No. 2,038,255 also discloses an external pipe communicating between two bellows where a specially moulded pipe coupling is provided which snap engages with the rack bar housing and to which the bellows are fitted with the result that the external diameter of the rack bar housing may be inconveniently enlarged by the attachment of the pipe coupling.
It is an object of the present invention to provide a rack and pinion steering gear assembly of the kind generally discussed above and by which the disadvantages of the prior proposals are alleviated whereby the flexible enclosure or bellows may be of a conventional form (that is without special mouldings or attachments to facilitate breathing) and which includes means through which the bellows can breather that is relatively inexpensive and simple to incorporate within the assembly and is relatively unobtrusive on the exterior of the assembly; the present invention also has as an object to provide a means for permitting the flexible enclosure or bellows to breathe, particularly (but not essentially) in a power assisted rack and pinion steering gear.